Insulating the underside of deck

9 years ago

Our deck has been updated to a three season porch. It is about three feet off the ground and has mesh stapled to the underside to prevent bugs from making their way in. A couple of years ago we purchased some carpet to place out there to try and warm the floor. I would like to ditch the carpet but the floor would be too cold on spring and fall days without some type of insulation. My question is: What type of insulation? After insulating should we put plastic and/or plywood? This will be a lay-on-your-back- in -the -landscape- rocks job so not fun but we are hopeful it will pay off.

Any advice you can provide would be greatly appreciated.

thank you.

Featured Answer

justretired
Original Author

9 years ago

Hmmm. The same contractor that built our house/deck did the construction on the three season porch for us. He did reinstall larger corner posts. The floor is treated lumber with a small bit of space in between each board. The underside of the floor is not exposed to weather as it is close to the ground, and there is heavy duty plastic under the existing landscape rock. I guess humidity could cause moisture?

So the best thing is not to insulate and just live with what we have? That would be OK, just thought I could make things look a bit better by eliminating the carpet.

Thank you for your response.

Comments (6)

live_wire_oak
9 years ago
You can do more harm than good insulating a space like that. It will hold moisture against the floor and rot it out. The proper approach to creating a room like that is NOT to enclose a deck. You need a proper foundation and stem walls, with moisture control at the ground level.
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millworkman
9 years ago
Humidity IS moisture, not a good idea as mentioned above.
justretired
Original Author
9 years ago
Yes. OK, not a good idea to insulate. We can live with that, plenty of other projects to do. Glad for the information.
worthy
9 years ago
Insulating a cold floor--whether it's a room over an unheated garage, a first floor over a crawl space or what was originally a deck--is certainly possible. In renoing his 100 year old home a friend of mine discovered that the kitchen was originally a porch that had over the years been enclosed; and two floors were added above. The key is proper air sealing.

See link below to how-to piece by building expert Martin Holladay of Green Building Advisor.

Here is a link that might be useful: How to Insulate a cold floor.

This post was edited by worthy on Tue, Oct 14, 14 at 16:46
energy_rater_la
9 years ago
nice link Worthy!

I agree with the foam sheathing attached
to the bottom of the joists. joist bays filled
with insulation.
I use R-30 (unfaced) in 2x6 joists and let it be compressed
by the foam sheathing on bottom of joists.
reduces it to about R-25, but but stays in full contact
with the flooring above.
caulk foam sheathing at perimeter of house, tape seams.

foam stops wind washing that robs the insulation of its
R-value, keeps batts in place, adds insulation value..

good diy project. oh and use button cap nails or
large head roofing nails to install foam sheathing.
putting large sheets of visqueen on the ground will
help you to slide around under there.

best of luck.